The Specific Aims for the next five years of our Merit Award include: (a) the continued development of practical methodology for the synthesis of purine and pyrimidine nucleoside analogs; (b) the continued synthesis (by the Liotta group) and evaluation (by the Schinazi group) of selected D- and L-nucleoside analog targets which possess favorable anti-HIV / HBV / HCV profiles with an emphasis on fluorinated derivatives; and (c) the continued development of a rational (de novo) drug design program aimed at the discovery of inhibitors of the RNA-dependent RNA polymerase (RDRP) of hepatitis C. In particular, we will continue to prepare ribonucleoside analogs that are potential inhibitors of the HCV polymerase. Our design principles include: (a) the presence of one or more oxygens or their surrogates (e.g., fluorine) at the 2'- and 3'-position to mimic key recognition elements in RDRP substrates and (b) attempt to maintain a near- eclipsed relationship between the 2'- and 3'-substituents consistent with conformations observed in related ribonucleotide / polymerase crystal structures. We will also continue our work on carbocyclic nucleoside analogs. Because the cyclobutyl ring in a steric sense represents a rigid spacer, we can use these compounds to address several important questions. First, what three-dimensional structural elements are required for nucleoside analogs to be good substrates for the endogenous kinases needed to convert them to their corresponding triphosphates? At present, little is known about this important question? Second, can we appropriately decorate cyclobutyl rings with functionality that will not only make them active inhibitors of the HIV and HBV RT functions and the HCV polymerase, but will also be less sensitive to mutations that contract the size of binding sites (e.g., M184V) because of their compact nature? The development of HIV entry inhibitors is an important goal because these compounds should, in principle, be able to suppress all viral strains with post-entry mutations. The development of CXCR4 antagonists is particularly important since T-tropic viral strains dominate the late stages of HIV infection where most of our current therapies provide only moderate to marginal benefits. The compounds that we have prepared to date are not only potent and selective CXCR4antagonists, but also exhibit no obvious toxicities both in vitro and in vivo. We will explore the potential of using CXCR4antagonists as potential treatments for HIV infection.